Ultra-Low Frequency (ULF) signals, ranging from 300 Hz to 3 kHz, can penetrate through conductive media, such as water, metal, soil, rock, and building materials, for hundreds of meters. Penetration is possible due to the relatively large skin depth in these materials, which grows as the carrier frequency is reduced. Very-Low-Frequency (VLF) signals, ranging from 3 kHz to 30 kHz, are typically used for long distance communications, such as between a base station and submarines located on the other side of world.
There is a tremendous amount of interest in the defense community for portable ULF/VLF transmitters for underwater, underground, and other communications. The current ULF technology employs large electromagnetic coils that weigh hundreds of pounds while consuming hundreds of kilowatts, which makes them impossible for portable deployments.
Due to the long wavelengths and unique applications, little advancement has been made in device miniaturization and integration in the ULF/VLF bands. Thus, there are still no portable ULF/VLF transmitters available. For many defense applications, the ability to communicate underwater, underground, or through cave walls, would be of great interest.